Implement Markdown definition lists (+ migration) (#20333)

Implements definition lists Markdown extension adopted in a few
implementations including:
* [Pandoc](
  https://pandoc.org/MANUAL.html#definition-lists)
* [kramdown](
  https://kramdown.gettalong.org/quickref.html#definition-lists)
* [PHP extra Markdown](
  https://michelf.ca/projects/php-markdown/extra/#def-list)

Also affected files have been migrated.
RST definition lists are turned off for Markdown: this solves the
problem of broken formatting mentioned in
https://github.com/nim-lang/Nim/pull/20292.

compiler/aliases.nim

@@ -77,24 +77,29 @@ proc isPartOf*(a, b: PNode): TAnalysisResult =
   ## cases:
   ##
   ## YES-cases:
+  ##  ```
   ##  x    <| x   # for general trees
   ##  x[]  <| x
   ##  x[i] <| x
   ##  x.f  <| x
+  ##  ```
   ##
   ## NO-cases:
+  ## ```
   ## x           !<| y    # depending on type and symbol kind
   ## x[constA]   !<| x[constB]
   ## x.f         !<| x.g
   ## x.f         !<| y.f  iff x !<= y
+  ## ```
   ##
   ## MAYBE-cases:
   ##
+  ##  ```
   ##  x[] ?<| y[]   iff compatible type
   ##
   ##
   ##  x[]  ?<| y  depending on type
-  ##
+  ##  ```
   if a.kind == b.kind:
     case a.kind
     of nkSym: